A 4-cycle engine generally has a valve system in which valve springs disposed between spring seats of a cylinder head and spring retainers bias, in a closing direction, intake and exhaust valves for opening/closing intake and exhaust valve openings that communicate with a combustion chamber, and in which camshafts drive the intake and exhaust valves to open/close.
Further, in some engine capable of rotating at high speed, the aforesaid valve spring has a double structure consisting of an inner spring and an outer spring disposed coaxially outside the inner spring. In arranging such inner and outer springs, spring constant and spring length thereof are determined depending on a set load, the maximum load at the time of lifting, and so on.
Incidentally, since a height dimension of the cylinder head in a cylinder axial direction is influenced by the spring length of the valve spring, the spring length is preferably made as short as possible.
However, since the spring length of the valve spring is determined depending on the set load, the maximum load at the time of lifting, and so on, there naturally is a limit in shortening the spring length. In the aforesaid double structure consisting of the inner and outer springs, since a spring seat of the outer spring is large in outer diameter, the spring seat needs to be disposed on an upper side in order to secure a necessary wall thickness of, especially, an intake port, and therefore, there is a concern that the height dimension of the cylinder head accordingly increases.
The present invention was devised in view of the conventional circumstances described above, and it is an object of the present invention to provide a valve spring support structure of an engine capable of reducing the height dimension of a cylinder head yet securing a necessary spring length of a valve spring.